1. Field of the Invention
The present invention generally relates to a fiber Bragg grating sensor system. Particularly, it relates to a fiber Bragg grating sensor system, which improves wavelength stability and repeatability of a wavelength tunable laser, and increases system accuracy by removing polarization dependency.
2. Description of the Related Art
A fiber Bragg grating is to induce a periodical modulation of the refractive index in the core of an optical fiber. The fiber grating is characterized in that it reflects only light within a narrow bandwidth (generally, 0.1˜1 nm) centered at Bragg wavelength, which meets the Bragg conditions and otherwise transmits. The Bragg wavelength is varied according to a change in temperature of the fiber Bragg grating and stress applied thereto. Thus, many fiber grating sensors have been developed using the above characteristics in order to measure any perturbations in temperature, strain, or stress.
An advantage of these fiber grating sensors is to measure an absolute value of physical properties such as temperature, strain etc. so that the sensors are immune to the electromagnetic interference. Because the optical fiber consists of silica glass (SiO2), they are a small and light electric insulator. Therefore, they can be installed and inserted to a measured object without affecting the function of the object. Moreover, in a fiber itself, light signals can be traveled to a long distance with little loss. Thus, the fiber grating sensors are easy to telemeter.
Another advantage of the fiber grating sensors is easy to multiplex a lot of grating sensors along one or several fibers installed at several places in order to measure simultaneously. That is, it is easy to apply a wavelength-division multiplexing method by designing each of fiber gratings to have a different reflective wavelength with each other in order that each of wavelengths does not overlap under sensor operation. With this method, a function of a quasi-distributed sensor can be carried out by placing each of the sensors at arbitrary positions separated by a few millimeters up to tens of kilometers.
The first proposed method for embodying a fiber Bragg grating sensor system is to measure a fiber grating wavelength with a broadband source, which has a wide linewidth, and a wavelength tunable filter [A. D. Kersey, T. A. Berkoff, and W. W. Morey, “Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter”, Optics Letters, Vol. 18, pp. 1370-1372, 1993]. The method tunes an applied voltage, thereby changing wavelengths of the tunable filter being at least those of the fiber gratings.
Employing the specific relationship between the applied voltage and the tuning wavelength of the tunable filter, the method measures the wavelengths of the fiber gratings by detecting a driving voltage when peak of fiber grating reflection signals appears.
Another method comprises a wavelength tunable laser used as a light source [S. H. Yun, D. J. Richardson, and B. Y. Kim, “Interrogation of fiber grating sensor arrays with a wavelength-swept fiber laser”, Optics Letters, Vol. 23, pp. 843-845, 1998].
First, the method tunes wavelengths of the laser being at least those of the fiber gratings with a wavelength tunable filter as a function of time.
With the specific relationship between scan time and the tuning wavelengths, the method measures the wavelengths of the fiber gratings by detecting the time when peaks of fiber grating reflection signals appear.
However, in the above-described methods, the wavelength of the practically used tunable filter is not varied linearly according to the applied voltage. Also, there is a problem of deteriorating accuracy and repeatability in long-term measurement due to the wavelength drift of the filter caused by hysteresis or temperature change. Moreover, in those methods an error caused by polarization dependency and the solutions thereof are not disclosed.
Accordingly, it was required for a wavelength reference and a compensating method to solve the above-illustrated problem so as to measure the Bragg wavelength with high accuracy and repeatability and the following methods have been developed. In the following, the principle and problems of the methods will be discussed.
FIGS. 1 through 3 schematically illustrate prior arts for obtaining accuracy of the fiber Bragg grating sensor system. FIG. 1 describes the method disclosed in U.S. Pat. No. 6,097,487 by Kringlebotn et al., FIG. 2 illustrates the method disclosed in U.S. Pat. No. 6,327,036 by Bao et al., and FIG. 3 describes the method disclosed in U.S. Pat. No. 6,449,047 by Bao et al.
Referring to FIG. 1, in the method disclosed in U.S. Pat. No. 6,097,487, a fiber ring laser is used as a light source 1, and wavelength of the laser is changed with a wavelength tunable filter 2 in a laser resonator. A gain medium 3 is pumped by a pumping laser diode 5 via a wavelength-division multiplexer 4. An optical isolator 6 is used for a unidirectional light transmission, for example, only counterclockwise. A laser output of varying wavelength with a narrow linewidth is outputted via a first coupler 7, and then split at a second coupler 8 into two directions. One light from the second coupler 8 is reflected from the fiber grating 9 and the reflected light is directed onto a first photo detector 10, and the other light is transmitted onto a second photo detector 13 via a Fabry-Perot filter 11 and a band pass filter 12. A third coupler 14 is inserted between the second coupler 8 and a fiber-grating array 9. Most of the light from the light source 1 is transmitted onto the fiber-grating array 9 via the third coupler 14, and one of these gratings becomes a reference grating used as an absolute reference wavelength.
Signals detected from the first and second photodetectors 10, 13 are simultaneously sampled, processed, and compared in a signal processing unit 16.
As described above, in this method the Fabry-Perot filter 11 and a reference fiber grating 15 are used for generating a wavelength reference to measure a laser wavelength. Since lights are transmitted via the Fabry-Perot filter 11 only with equally spaced and known frequencies, the frequencies are used as the wavelength reference. That is, a magnitude of the wavelength shift of the laser can be known by measuring output signals of the Fabry-Perot filter 11.
The reference fiber grating 15 is used to calculate an absolute wavelength of the laser. Although relative laser wavelengths can be calculated by the use of the previous output signals of the Fabry-Perot filter 11, the absolute laser wavelength cannot be calculated. Thus, location of reflected peak from the reference fiber grating 15 with the known wavelength is measured and used as the absolute wavelength reference for figuring the absolute laser wavelength of the laser as a function of time.
In brief, it is the method in which the laser wavelength shifts are calculated by the use of the reference fiber grating 15 and the peak signals of the Fabry-Perot filter 11 and thereby measuring the wavelengths of the fiber grating sensors.
However, if the wavelength of the reference fiber grating 15 varies in temperature, etc., the described method is to have an error in measuring the wavelength. Therefore, the reference fiber grating 15 should have temperature stability regardless of the surrounding environments.
Referring to FIG. 2, in the method disclosed in U.S. Pat. No. 6,327,036, the reference fiber grating I and the Fabry-Perot filter 2 are used in order to form a reference wavelength as shown in FIG. 1.
However, in this method a wavelength of the reference fiber grating 1 is matched to a specific wavelength transmitted via the Farbry-Perot filter 2, thereby magnifying or diminishing the specific wavelength to distinguish the specific wavelength from the other peak wavelengths. The distinguished specific wavelength is used as the absolute wavelength reference. With this method, a wavelength shift of an input light, i.e. a first light source 3 can have been measured accurately. However, this method has a drawback of requiring an additional light source, i.e. a second light source 4. Also, the method requires a time-divisional technique using an optical switch 5 and a splitter 6 in order to generate a reference wavelength and to measure a wavelength of the first light source 3 alternately. The reference wavelength is generated by introducing a light from the second light source 4 to a wavelength scanner 9 using a wavelength tunable filter 8 via the Fabry-Perot filter 2, the reference fiber grating 1, and a band pass filter 7. Accordingly, in this method, there is a drawback of a limitation in speed for obtaining wavelength information of the input light signal due to generation of the wavelength reference employing the time-division method.
Referring to FIG. 3, in the method disclosed in U.S. Pat. No. 6,449,047, a reference fiber grating 1 and a Fabry-Perot filter 2 are used as shown in FIG. 2 to generate a reference wavelength of a laser, where the wavelength of the reference fiber grating 1 is matched to a specific wavelength transmitted via the Fabry-Perot filter 2. A rapidly wavelength-swept laser as a light source 3 was applied and a wavelength tunable filter, in which voltage would control the transmitted wavelength of the filter to change the wavelength of the laser, was employed. A portion of the laser output is introduced into a first photodetector 5 via a first coupler 4, the Fabry-Perot filter 2, and the reference fiber grating 1 in that order. In a signal processing unit 6, the wavelength of the laser is calculated analyzing signals from the first photodetector 5. Most of the laser output is injected into a fiber grating sensor array 7 via first and second couplers 4, 5 sequentially. The reflected signals from the fiber grating sensor array 7 are directed onto a second photodetector 8, and a wavelength of light reflected from the fiber grating array 7 is calculated in the signal processing unit 6 by comparing these signals with the incident signals on the first photodetector 5. With this method, non-linearity can be removed over the whole wavelength range, and wavelength accuracy of the system can be obtained. Here, the term “accuracy” refers to the difference between the wavelength measured by a sensor system and the true one.
The prior arts illustrated from FIGS. 1 through 3 are related to methods for generating the reference wavelength and continuously monitoring wavelengths of the light source with the reference wavelength, thereby improving wavelength accuracy of the fiber grating sensor system.
However, polarization dependency, one of the essential factors causing accuracy problems in measuring, can be present in the fiber Bragg grating, the Fabry-Perot filter, the photodetector, and so on. Particularly the polarization dependency of the fiber grating in itself can generate significant errors in measuring a change in strain. The polarization dependency of the fiber grating denotes a phenomenon that the wavelength of reflected light varies according to polarization of the incident light to the grating. If the fiber grating has an internal birefringence or a birefringence is induced by an ambience influence such as transverse stress, bending of an optical fiber, and so forth, the phenomenon occurs. In the practical fiber grating sensor system, during a light is transmitted up to tens of kilometers through the optical fiber, polarization states of the light are randomly changed according to perturbations. As a result, the wavelength is changed according to the polarization states, thereby errors occurring in measuring. For solving this problem, there is either a packaging method of the fiber grating not to sustain transverse stress or a method for utilizing polarization-maintaining fibers to preserve the polarization states. However, these methods are expensive and non-effective. Therefore, the following method has been developed to effectively solve this problem.
FIG. 4 schematically illustrates a method for stably measuring a change in transverse strain by reducing the polarization dependency, disclosed in U.S. Pat. No. 6,363,180 to Yamate et al.
Referring to FIG. 4, in the method disclosed in U.S. Pat. No. 6,363,180, characteristics that wavelengths of light reflected from the Bragg grating become different from each other according to the polarization of light are employed in measuring a change in transverse strain. If transverse stress is applied to the fiber grating, birefringence is induced, and two reflective peaks according to the birefringence are produced. That is, the two peaks corresponding each of eigen-polarizations are generated, the peak-to-peak separation is proportional to the magnitude of the transverse stress. By the way, if the input polarization states are varied or a detection system for light signals has the polarization dependency, stable signals cannot be obtained in measuring. Therefore, this method has improved the stability of the sensor system either by controlling polarization of a light source 2 being incident to a sensor 1 with a polarizer 3 and a controller 4 or by adjusting the polarization of the light, which is reflected from the sensor 1 and then introduced to a detection system 6 via a coupler 5 with a polarization rotator 7. The detection system 6 includes a recorder 8 and a wavelength tunable filter 9. Since a Fabry-Perot filter for analyzing reflected wavelength from the sensor had polarization dependency, a polarization scrambler 10 was applied to remove this polarization dependency.
However, in case of use of a polarization controller or polarization scrambler, the measurement should be carried out for enough time and the result therefrom should be averaged to remove polarization dependency. Accordingly, the fast polarization scrambler, which may be very expensive and increase insertion loss, is required for fast measurement of strain.
Accordingly, it is noted that problems of the wavelength stability and the polarization dependency should be solved to fabricate the practically useful fiber Bragg grating sensor system.